During the production of pulps by fine mechanical means (that is utilizing refiners (defibrators)), a portion of the material which is produced is very fine, typically having a size under about 200 mesh. The drainage characteristics of this fines fraction is poor, in fact it is so poor that it adversely affects the drainage characteristics of the entire pulp stream even though it typically makes up only about 10-20 percent of the entire pulp stream. Because of the poor drainage characteristics it is impractical to use displacement bleaching or washing techniques, with the result that the pulp can be bleached only to brightness levels which are less than theoretically possible.
It is highly desirable to be able to use displacement bleaching and washing techniques in the treatment of mechanical pulp since displacement bleaching has been shown to be a way to obtain the maximum removal of water and metal ions contained within the pulp lattice, using a minimum amount of water or bleaching chemical. Displacement bleaching allows a maximum bleaching to take place, with a minimum amount of bleaching chemical.
Because of the poor drainage characteristics of conventional mechanical pulps, two or more stages of bleaching have been proposed in order to give a higher brightness, but such stages result in improved bleaching only up to the point that the level of metal ions, and like contaminants within the pulp, rise to an unacceptable level. Therefore in present commercial practice the brightness ceiling for mechanical pulps is on the order of 75 GE. If this level could be increased about 7-10 points, mechanical pulp would be acceptable in many markets where it is now precluded. However since hydrogen peroxide, the conventional bleaching chemical utilized in mechanical pulp bleaching, is very expensive, this additional increase in brightness must be achieved with an acceptable peroxide consumption.
According to the present invention, it is possible to produce mechanical pulp having a higher brightness than the present practical ceiling of 75 GE, and it can be produced in a cost-effective manner. According to the present invention, a maximum increase in brightness can be obtained for a minimum consumption of hydrogen peroxide, by improving the drainage characteristics of the majority of the mechanical pulp so that it can be treated by displacement washing and/or bleaching.
According to the present invention, a first stream of mechanical pulp is separated into a minor fines fraction and a major fiber fraction. The separation may be accomplished utilizing a screen, or a pair of centrifugal separators (cyclones), the first cyclone separating the major fiber fraction from the fines, and the second connected to the fines discharge from the first and separating the fines from a fluid containing steam. A portion of the fluid containing steam which is separated by the second separator may be recycled to the inlet to the first separator. The major fiber fraction may be subjected to displacement bleaching utilizing a suitable bleaching chemical, such as hydrogen peroxide, to produce a bleached mechanical pulp having higher brightness per unit of hydrogen peroxide consumed than if the stream of mechanical pulp were bleached without the separation stage. The fines fraction also is preferably bleached utilizing hydrogen peroxide, with non-displacement bleaching techniques.
It is the primary object of the present invention to provide a method and apparatus for improving the drainage characteristics of mechanical pulp to ultimately allow the production of bleached mechanical pulp with a higher brightness and/or with a maximum brightness per unit of hydrogen peroxide consumed. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.